1. Field of the Invention
The present invention relates to the process of ion implantation, and particularly to an apparatus and method for implanting ions of different elements into metal, quartz, and semiconductor wafers ranging in diameter from 76 mm to 450 mm in the fabrication of very large scale integrated circuits. The apparatus and method of the present invention provides for the very high efficiency large scale manufacture of various types of semiconductor devices such as solar cells and integrated circuits and for the surface modification of metal and dielectric wafer surfaces.
2. Description of the Related Art
Ion implantation plays an essential role in the synthesis of thin films, the epitaxial growth of thin films, and the manufacturing of semiconductor devices. Ion implantation makes it possible to synthesize high quality thin films, to grow high quality thin films, and to manufacture semiconductor devices with a very high degree of precision and accuracy, and with high yields. Ion implantation equipment typically consists of an ion implanter and an end station or target unit that houses the articles to be processed. Ion implantation is achieved either by beam scanning of the target, which involves the electrostatic or magnetic movement of the beam across the target, or mechanical scanning of the target, which involves moving the target through the beam.
Mechanical scanning of the target through the beam results in a more uniform implanting of the target than does beam scanning across the target; therefore, mechanical scanning is the preferred technique. The implantation ions of the ion beam are mass-selected using a magnet housed in the ion implanter. Mass selection of the implanted ions enables the implantation of specific ionic species. For example, C+ ions are mass-selected from the other ionic species present in the ionized source gas used to obtain C+ ions for implantation into the target element.
The present invention is a method and an apparatus for automatically handling wafers having different configurations, such as silicon wafers, silicon dioxide wafers, metal wafers, other dielectric wafers and other semiconductor wafers. These wafers are commonly referred to as substrates.
Ion beam processing of substrates having diameters larger than 150-200 mm is cumbersome, inefficient, and expensive. There are a number of problems associated with the ion beam processing of substrates, particularly silicon substrates, using conventional ion beam processing equipment. For example, the quality of thin films synthesized using either ion beam deposition or ion beam epitaxy is considerably better when the synthesis occurs at temperatures higher than room temperature. On the other hand, the manufacture of semiconductor devices and integrated circuits require ion beams having values in the range of hundreds of microamperes to a few milliamperes.
Ion implantation is used in semiconductor device manufacturing to introduce dopants into a semiconductor substrate; for example, a silicon substrate, to alter the conductivity of portions of the semiconductor substrate. Ion beams having those magnitudes of currents require accelerating voltages in the range of 200-300 keV. However, at such high magnitudes of ion beam current and accelerating voltage, heating of the silicon substrate, which is extremely undesirable, is unavoidable and inevitable.
The present invention utilizes a spiral graphite heater to heat the silicon substrates during the ion beam deposition or the ion beam growth of thin films and a cooling means to remove heat generated during the ion beam fabrication of integrated circuits. Presently, the primary commercial use of ion implantation is in the manufacture of large scale integrated circuits (LSIC) chips. Implanting conductivity modifying chemical impurities into semiconductor wafers is a significant part of the process for manufacturing semiconductor devices such as large scale integrated circuit chips.
The density of integrated circuits and their speed of operation are very dependent upon tight control of the profile of doped regions in a wafer, that is, regions to which substantial concentrations of conductivity modifying impurities have been added. The tight control of wafer doping is best achieved using ion implantation techniques and equipment. Ion implantation doping methods result in improved very large scale integration (VLSI) by making more efficient use of the wafer area, shortening interconnects between devices, producing smaller geometries, and reducing noise.
Ion implantation is the doping process of choice because of the kinds of doping profiles, concentrations, and lateral geometries required on a VLSIC wafer. Only ion implantation is capable of providing the uniformity of doping that is critical in the fabrication of smaller geometry devices. Furthermore, doping uniformity across the wafer and repeatability from wafer to wafer, which is achievable with ion implantation, dramatically improves fabrication yields of high density devices.
Silicon ingots of diameters up to 300 mm are now available; however, most conventional ion implantation equipment is designed to accommodate substrates, that are cut from an ingot, of substantially smaller diameter (150 mm or less). The processing of substrates with diameters in excess of 150 mm by conventional ion implantation equipment requires costly modifications of the equipment.
The mechanical scanning cylindrical carousel apparatus can accommodate substrates with diameters in excess of 150 mm; however, the carousel apparatus has a low capacity being able to accommodate no more than 50 substrates with diameters of 300 mm, which results in a low throughput. The widely used spinning disc apparatus requires substrates with diameters in excess of 250 mm to have the same capacity as the cylindrical carousel apparatus; however, the spinning disc apparatus is not able to accommodate substrates with such a large diameter. In addition, scanning errors arise as a result of the radial translation of the discs (substrates) through the ion beam which results in dose nonuniformity in the radial direction.
The ion implantation apparatus of the present invention readily accommodates substrates with diameters in excess of 150 mm; therefore, a high degree of integration in the fabrication of integrated circuits is possible which results in a reduction in the cost of fabricating the integrated circuits. The present invention is capable of accommodating a large number of substrates with diameters in excess of 150 mm resulting in a method and apparatus with a high throughput and an excellent yield. The prior art does not describe any method or apparatus with the capabilities of the present invention.
U.S. Pat. No. 4,975,586 issued on Dec. 4, 1990 to Andrew M. Ray describes the end station of an ion implantation apparatus that includes a rotatable wafer support. The wafer being processed is rotated during and between implants; therefore, the wafers are implanted at variable implant angles. The wafer support is mounted on a first housing which is supported for rotation by a hub assembly extending through the wall of the end station vacuum chamber and is driven by a stepper motor through a drive belt and sheave system.
Rotation of the support about its own axis is provided by a stepper motor mounted on the rotating housing and connected to the wafer support by means of a drive system within the housing such that the two drive systems are operable independently of each other. The ion implantation apparatus of the Ray patent has a low throughput and a low capacity because the processing chamber can only accommodate a single wafer.
U.S. Pat. No. 4,948,979 issued on Aug. 14, 1990 to Yasao Munakata et al. describes a vacuum device that consists of a vacuum working chamber, a vacuum prechamber, and a communicating member that connects the vacuum working chamber and the vacuum prechamber. The communicating member is an independent member that is inserted between both vacuum chambers when needed. The vacuum working chamber contains an electronic gun for processing a substrate disposed in the vacuum chamber. The electronic gun emits an energy beam to form a descriptive pattern on the surface of the substrate.
The Munakata et al. patent describes a method of processing a substrate wherein the substrate to be processed is initially placed in the prechamber. The prechamber undergoes a vacuum pumpdown to evacuate the prechamber while the vacuum condition of the working chamber is maintained. The substrate is then transferred from the prechamber to the working chamber through the communication member without breaking vacuum conditions.
The substrate is then processed using the energy beam from the electronic gun. After processing is completed, the substrate is removed from the working chamber and returned to the prechamber through the communication member. The. vacuum condition of the prechamber is then released and the substrate is removed from the prechamber when the interior of the prechamber reaches atmospheric condition.
The method and vacuum apparatus of the Munakata et al. patent pertains to electron beam processing of a substrate which is conventionally used to pattern a resist coated on the surface of an underlying substrate and not to ion implantation. The method and apparatus of the Munakata et al. patent has a high turn-around time compared to the present invention because the working chamber and the prechamber are separate and must be repeatedly connected and disconnected. In addition, the method and apparatus described by the Munakata et al. patent has a low throughput because the vacuum apparatus of the Munakata el al patent processes only one substrate at a time.
A method and apparatus for ion implanting semiconductors using an apparatus that comprises a support rotor for carrying the target wafers on support bases mounted on arms extending radially from a core structure of the rotor is described in U.S. Pat. No. 5,124,557 issued on Jun. 23, 1992 to Derek Aitken. The apparatus of the Aitken patent comprises a support rotor comprising a central core structure or hub, numerous support bases arranged in a circular configuration around the hub with each support base being mounted on a radial support arm. The support rotor is mounted for rotation on an axle and rotated about an axis by a spin motor which in turn is mounted on a shaft. The target wafers are rotated by the rotor through the ion beam to produce scanning across the target wafers.
None of the above inventions and patents, taken either singly or in combination, is seen to describe the instant invention as claimed. Thus, an economical, high capacity target presentation unit for the ion beam processing of large semiconductor substrates that solves the aforementioned problems is desired.
Accordingly, it is a principal object of the invention to provide a method and apparatus for the ion implantation processing of semiconductor substrates that has a high throughput, a high density of integration, and a high yield.
It is another object of the invention to provide a provide an ion implantation processing apparatus that has an economical, compact design that is both operationally efficient and cost effective.
It is a further object of the invention to provide a method and apparatus that optimizes the deposition of thin films and the ion implantation of semiconductor wafers.
Still another object of the invention is to provide a method and apparatus that prevents the detrimental effects due to unwanted heating that occurs during implantation.
It is an object of the invention to provide improved elements and arrangements thereof for the purposes described which is inexpensive, dependable and fully effective in accomplishing its intended purposes.
To meet the above objectives, the present invention provides a method and apparatus that comprises processing substrates in a multi-chamber apparatus. The multi-chamber apparatus comprises a central vacuum chamber where the substrates are processed and two symmetrically disposed lateral vacuum chambers. One of the lateral vacuum chambers is the loading chamber into which the substrates mounted on cassettes are loaded, whereas the second lateral vacuum chamber is the discharge chamber into which the substrates are transferred after processing.
The cassettes with mounted substrates are conveyed from one vacuum chamber to another by means of several screw conveyors and a looped chain conveyor. The screw conveyors advance the cassettes forward in the loading chamber and in the discharge chamber. The cassettes are advanced in the implantation chamber by means of a looped chain conveyor that moves the cassettes in a direction that is perpendicular to the direction of movement of the cassettes in the loading and discharge chambers.
The present invention processes large silicon and quartz plates both efficiently and economically. The present invention has a large capacity and a high throughput being able to process a large number of silicon or quartz plates in a relatively short period of time. The compact design of the present invention results in much lower costs both in the cost of materials to manufacture the present invention, and in the cost of operating and maintaining the present invention.
The present invention makes use of directing rods disposed within the loading and discharge chambers to transport and guide the silicon or quartz plates mounted on the cassettes through the loading and discharge chambers, and a pair of runners disposed within the processing chamber to guide the silicon or quartz plates through the processing chamber; that is, the silicon or quartz plates are moved from the loading chamber to the processing chamber to the discharge chamber in a continuous manner. At no time after the silicon or quartz plates are loaded into the loading chamber and transported between the loading, implantation, and discharge chambers of the present invention is vacuum ever broken; therefore, the silicon or quartz plates are completely processed using a single vacuum pumpdown.
The implantation chamber of the present invention has a spiral graphite heater that is used to heat the substrates to the appropriate deposition temperature when the multi-chamber apparatus of the present invention is used to deposit thin films using the process of ion beam deposition, for example, a diamond-like carbon film. The implantation chamber of the present invention also has a cooler to cool the substrates during ion beam implantation to prevent the detrimental effects to integrated circuit performance due to unwanted diffusions caused by the heating of the substrates during the ion implantation process.
The ingenious simplicity and straightforwardness of the method of the present invention and the economy of design of the target presentation apparatus of the present invention result in increased productivity at reduced cost.
These and other objects of the present invention will become readily apparent upon further review of the following specification and drawings.